Syntax
------

label: <construct>
[label]

Labels are MariaDB identifiers which can be used to identify a BEGIN ... END
construct or a loop. They have a maximum length of 16 characters and can be
quoted with backticks (i.e.., `).

Labels have a start part and an end part. The start part must precede the
portion of code it refers to, must be followed by a colon (:) and can be on
the same or different line. The end part is optional and adds nothing, but can
make the code more readable. If used, the end part must precede the
construct's delimiter (;). Constructs identified by a label can be nested.
Each construct can be identified by only one label.

Labels need not be unique in the stored program they belong to. However, a
label for an inner loop cannot be identical to a label for an outer loop. In
this case, the following error would be produced:

ERROR 1309 (42000): Redefining label <label_name>

LEAVE and ITERATE statements can be used to exit or repeat a portion of code
identified by a label. They must be in the same Stored Routine, Trigger or
Event which contains the target label.

Below is an example using a simple label that is used to exit a LOOP:

CREATE PROCEDURE `test_sp`()
BEGIN
 `my_label`:
 LOOP
   SELECT 'looping';
   LEAVE `my_label`;
 END LOOP;
 SELECT 'out of loop';
END;

The following label is used to exit a procedure, and has an end part:

CREATE PROCEDURE `test_sp`()
`my_label`:
BEGIN
 IF @var = 1 THEN
   LEAVE `my_label`;
 END IF;
 DO something();
END `my_label`;

URL: https://mariadb.com/kb/en/labels/